Modeling of Voltage Hysteresis and Relaxation of HEV NiMH Battery

SOC (State of Charge) estimation-based battery management is essential for HEV (Hybrid Electric Vehicle) applications. The SOC can be estimated by the relationship between the OCV (Open Circuit Voltage) and SOC, which is based on the Nernst equation, in combination with Coulomb counting. However, voltage hysteresis and relaxation make measurement of the OCV difficult. The OCV measured after charge (discharge) is higher (lower) than the OCV estimated by the Nernst equation, and relaxes slowly with a time constant from a few minutes to a few hours. In this paper, to express voltage hysteresis and relaxation, a three-layer model for nickel active materials is proposed. A voltage gap between the surface and inside of the nickel active material is introduced, and voltage hysteresis is modeled with the surface partial battery governing the OCV of the whole battery. Voltage relaxation is also expressed as equalization between the surface layer and the relaxation layer with a higher internal resistance. The static and dynamic behavior of the proposed model is confirmed through experiments with a 7.2-V 6.5-Ah NiMH battery module. (C) 2011 Wiley Periodicals, Inc. Electr Eng Jpn, 175(1): 1-7, 2011; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/eej.21102